Reduction in carbon dioxide emissions has been strongly desired in recent years for environmental protection. The automotive industry has placed great expectations on the introduction of electric vehicles (EVs) or hybrid electric vehicles (HEVs) to reduce carbon dioxide emissions and has been extensively developing secondary batteries for driving the motors, the key to practical use of these vehicles. Secondary batteries that have received attention include lithium ion batteries, which have high energy density and high output power density.
A typical lithium ion secondary battery includes an electrode composed of a positive electrode current collector with a positive electrode active material applied thereto together with a binder and an electrode composed of a negative electrode current collector with a negative electrode active material applied thereto together with a binder. A bipolar battery includes a bipolar electrode composed of a current collector with a positive electrode layer on one side and a negative electrode layer on the other side. The positive electrode layer is formed by applying a positive electrode active material together with a binder to one side of the current collector. The negative electrode layer is formed by applying a negative electrode active material together with a binder to the other side (see Patent Literature 1, for example).
In Patent Literature 1, paste for forming electrodes is applied at a thickness of about 25 μm. Increasing the proportion of the positive electrode material and the negative electrode material in a battery is known as a way to increase the energy density of the battery. Patent Literature 2 discloses a method for increasing energy density of a battery, in which the film thickness of the electrodes is increased to reduce the relative proportion of the current collector and the separator.